The present invention relates to a pile system for securing an offshore structure and, more particularly, relates to a compliant pile system for securing a guyed tower offshore platform and supporting the net vertical weight thereof when said guyed tower is installed in a body of water.
New offshore structures recently have been proposed for recovering hydrocarbons from marine deposits which underlie great depths of water. One such offshore structure is a compliant platform known in the art as a "guyed tower" platform. Basically, a guyed tower is a trussed structure of uniform cross-section that rests on the marine bottom and extends upward to a deck supported above the surface. The structure is held upright by multiple guylines which are spaced about the trussed structure. The structure is "compliant", e.g. tilts, in response to surface wave or wind forces, thereby creating inertial forces which counteract the applied forces. These counteracting forces aid in reducing total forces transmitted to the platform's restraints.
While various geometric cross-sections may be used, the main truss of a typical guyed tower structure normally has four, equally-spaced legs connected together with conventional triangularly-arranged bracing members.
Previously proposed guyed towers have relied upon either a truss-reinforced shell foundation, called a "spud can", or piles to secure the structure in position and, more importantly, to carry the net vertical weight of the structure. The spud can provides a pivot point for the tilting of the structure. Since the structure rests directly on the marine bottom, the spud can serves primarily to transmit the axial load to the marine bottom in bearing capacity. Piles, on the other hand, extend from the connection of the pile to the platform (referred to as the "pile-platform" connection) through pile guides spaced along the length of the structure into the marine bottom. Piles support the structure by transmitting axial load as well as shear loads into the marine bottom.
Pile systems normally require multiple pile members which, due to available space, necessitates the placement of some or all of the main piles eccentric to the axis of tilt of the structure. Due to this eccentricity, the sway or tilting motions of the compliant guyed tower structure impose deflections at the pile-platform connection (referred to as "pile-head" deflections) that result in substantial increases in the axial forces applied to the piles. When the axial forces due to the pile-head deflections are added to the axial loads in the piles due to the weight of the structure, deck, etc., the total axial loads imposed on the piles may become excessive.
Further, since these piles may extend from the marine bottom to the surface, they may pass through the "wave zone." This is the zone of water at and below the surface which is affected by the presence of surface waves. Each of the piles presents a drag surface against which the waves act, thereby increasing the overturning forces applied to the guyed tower. Accordingly, it may be desirable to reduce both the axial loads on the piles and the drag surfaces exposed in the wave zone.